Many products made of different materials with different properties are manufactured by sealing the different materials together. A seal between two different materials can be produced by welding, gluing, brazing, or similar processes. Brazing is the process of soldering two materials with different properties, such as ceramic and metal, together using a hard solder with a relatively high melting point. One can create a very strong and lasting seal between two different materials by employing the braze materials and methods described in the prior art. Many products benefit from brazing, including medical devices such as implantable microstimulators. A strong hermetic seal is required between ceramic and metallic materials of the outer case of some microstimulators.
Implantable microstimulators known as Bion® devices are characterized by a small, cylindrical housing which contains electronic circuitry for producing electric currents between spaced electrodes. The microstimulators are implanted proximate to the target tissue, and the currents produced by the electrodes stimulate the tissue to reduce symptoms or otherwise provide therapy for various disorders. Microstimulators often include valuable electronic circuitry, batteries, and other components that must be hermetically sealed within a secure case in order to protect the inner components of the microstimulator from damage by surrounding tissue and in order to protect a patient from harm caused by a malfunctioning microstimulator.
Radio-frequency powered and battery powered microstimulators are described in the art. See, for instance, U.S. Pat. Nos. 5,193,539 (“Implantable Microstimulator”); 5,193,540 (“Structure and Method of Manufacture of an Implantable Microstimulator”); 5,312,439 (“Implantable Device Having an Electrolytic Storage Electrode”); 6,185,452 (“Battery-Powered Patient Implantable Device”); 6,164,284 and 6,208,894 (both titled “System of Implantable Device for Monitoring and/or Affecting Body Parameters”). The '539, '540, '439, '452, '284, and '894 patents are incorporated herein by reference in their entirety.
Microstimulators that prevent and/or treat various disorders associated with prolonged inactivity, confinement or immobilization of one or more muscles are taught, e.g., in U.S. Pat. Nos. 6,061,596 (“Method for Conditioning Pelvis Musculature Using an Implanted Microstimulator”); 6,051,017 (“Implantable Microstimulator and Systems Employing the Same”); 6,175,764 (“Implantable Microstimulator System for Producing Repeatable Patterns of Electrical Stimulation”); 6,181,965 (“Implantable Microstimulator System for Prevention of Disorders”); 6,185,455 (“Methods of Reducing the Incidence of Medical Complications Using Implantable Microstimulators”); and 6,214,032 (“System for Implanting a Microstimulator”). The techniques described in these additional patents, including power charging techniques, may also be used with the present invention. The '596, '017, '764, '965, '455, and '032 patents are incorporated herein by reference in their entirety.
The various types of microstimulators known in the art, and other products in other arts, often employ brazing materials and methods to create hermetic seals for the cases that house the inner components of such devices. For example, U.S. Pat. No. 6,221,513, which patent is incorporated herein by reference in its entirety, describes methods for hermetically sealing ceramic to metallic surfaces and assemblies that incorporate ceramic to metallic seals. The '513 patent discloses a brazed butt joint, a brazed bevel joint, and a braze joint between a metal end cap and a ceramic open-ended cylinder. Another example, International Publication No. WO 00/56394, which publication is incorporated herein by reference in its entirety, describes a ceramic case assembly for a microstimulator. The '394 publication discloses a brazed butt joint, a brazed internal step joint for self-jigging, and a braze joint between a metal end cap and a ceramic open-ended cylinder. Yet another example, International Publication No. WO 00/56677, which publication is also incorporated herein by reference in its entirety, describes both a self-jigging bevel joint and a self-jigging internal step joint for a metal-ceramic braze bond. In a final example, U.S. Pat. No. 4,991,582, which patent is also incorporated herein by reference in its entirety, discloses a metal to machined ceramic braze bond using a self-jigging step joint.
Although the various types of hermetic seals and braze joints known in the art may be useful for microstimulators and other products, significant improvements upon these seals and joints are still possible and desirable, particularly relative to a braze joint creating a strong and safe hermetic seal that can be successfully produced on a consistent basis and in a cost effective manner.
For example, the '513 patent and the '394 and '677 publications are likely to suffer from the undesirable effects of braze material that exudes from the joint to the outer surface of a device case during assembly. When braze material exudes, during assembly, to the outer surface of the case, the material cools after the brazing process is complete to create a sharp metallic burr, e.g., on the outside surface of the device case. This burr, if not removed, could cause significant discomfort, damage, and injury to a patient when the microstimulator is implanted. Yet, removing the dangerous burr after braze assembly using any technique—including chipping, sanding, shaving, laser cutting or other method—is certain to increase the manufacturing time and cost and is very likely to compromise the strength of the braze bond.
Further, the '582 patent discloses a step joint between a metal member and a ceramic case, which ceramic case is machined to include a step that specifically fits in communication with the metal member at the joint. Machining a ceramic case often leaves residual cracks and weakens the case, especially where the wall of the ceramic case is thin.
Therefore, a need exists for a braze joint assembly that improves upon the prior art by providing a strong ceramic case at the braze joint and a means for inhibiting braze material exudation.